The inventors have identified a real need for scanning or digitizing closed books, documents which are fragile or difficult to separate, bound or in batches, manuscripts, painted or printed without having to be opened or handled. In fact, due to their extreme fragility, it is imperative for some works to be kept closed and shielded from any destructive electromagnetic radiation which might alter their contents (for example, high-energy ionising rays). Also, there can be a considerable gain in time in digitizing if the pages of the book do not need to be turned.
Accordingly, the present invention applies in particular to the digitizing of printed documents, bound or in batches, comprising a plurality of printed pages superposed on each other.
In general, a printed document comprises a substrate (paper pages) on which is deposited a material (ink) defining information (in the form of an image or text) to be captured or extracted.
For this information to be captured, it is essential to release sufficient contrast (that is, observe sufficient difference in transmission or reflection of the waves) between the substrate and the ink of the document in question. Therefore, the contrast constitutes an essential parameter for digitizing printed documents.
The resolution constitutes another essential parameter for digitizing printed documents, so as to ensure the faithfulness of the digitizing with the information contained in the document, and for example to make a reproduction of this document.
In fact, the spatial digitizing resolution of an imaging system is the distance between two points of the object to be digitized, such that this distance corresponds to the dimension of the smallest element of the image recorded by this imaging system. In other words, the spatial resolution of an imaging system defines the fineness of details which it is possible to record. The level of detail recorded is all the higher the finer the spatial resolution of the system.
The inventors have noticed that THz waves are particularly well adapted to the particular application described hereinabove, to the extent where they produce a sufficient level of contrast between the substrate and the ink, benefiting from spatial resolution compatible with the fineness of the characters to be acquired on the pages of the printed and bound documents.
According to the inventors, the THz waves are the only electromagnetic waves which simultaneously have:                sufficient penetration power of waves through the material constituting the substrate of the ink,        contrast between the substrate and the ink exploitable for extracting the information; and        non-ionising radiation enabling use in an unprotected environment        
The maximal theoretical spatial resolution Δl of an imaging system 3D limited by the phenomenon of diffraction is calculated according to the following formula:
      Δ    ⁢                  ⁢    l    =      0.61    ⁢                  ⁢          λ      ON        ⁢          (              Rayleigh        ⁢                                  ⁢        criterion            )      where λ designates the wavelength of the electromagnetic radiation in question in the imaging system and ON designates the digital opening of the imaging system.
In this way, the use of THz waves of wavelength between 30 μm and 3 mm enables an imaging system to reach maximal theoretical spatial resolution Δl between 36.6 μm and 3.66 mm, which is close to spatial resolutions relative to the standard size of information elements normally found on printed documents. In practice, the elements making up the characters or letters of a book are of a dimension of the order of a few hundreds of micrometers.
Current THz 3D imaging systems, an example of which is described in U.S. Pat. No. 7,119,339, do not attain this maximal theoretical spatial resolution, due to the fact that commercially available THz systems have a spatial resolution limited by their signal-to-noise ratio for the relevant frequency or by the dimension of the source and of the detector.
In an optical digitizing system it is known for the resolution of the detector (which corresponds to the physical size of the resolution element of the detector once converted to the object to be digitized by means of the enlargement of the optical system) to generally limit the spatial digitizing resolution of the system.
In particular, within the scope of the particular application described above, current imaging systems do not distinguish two consecutive pages of a book kept closed. For this, it is necessary for the system to provide spatial resolution of the improved digitized images in the dimension corresponding to the thickness of the book, relative to the spatial resolution obtained in the dimension of width or length of the book. Also, to correctly distinguish the characters printed on a page, it is therefore necessary to increase the spatial resolution of THZ 3D digitizing systems.